The present invention is concerned with electronically-controlled railway vehicle brake systems, and particularly to such systems in which electropneumatic application and release magnet valves are employed to regulate the friction brake forces in accordance with digital control signals generated by an electronic control unit.
In an earlier designed RT-5 type brake equipment, manufactured by the Westinghouse Air Brake Company, there is shown in FIG. 1 of the drawings a G-4 type pneumatic-operating unit having an N-2D magnet valve portion comprising low-capacity application and release magnet valves that operate to regulate the vehicle brake cylinder pressure via a high-capacity J-1 relay valve device, the latter being necessary to satisfy the high-capacity air requirement of the brake cylinder. Since these application and release magnet valves are low-capacity, the emergency and wheel-slip control functions, which require fast response, must be handled by additional pneumatic components; such as, an A-2 reduction relay valve and a B-3-C decelostat valve. Thus, a large number of pneumatic components are required to provide the service, emergency, and wheel-slip functions. This high-component count results in high weight, low reliability, high cost, slow response, and reduced accuracy. In addition, such an arrangement is complicated to adapt to different transit applications, since a great deal of the system logic is performed by the pneumatics and the associated piping.
Moreover, in FIG. 1, a pressure-feedback transducer, located between the magnet valves and relay valve, provides the electronic control unit with a signal representative of the brake cylinder pressure, whereby the electronic unit compares the combined dynamic and friction brake levels with the brake command and operates the magnet valves to regulate the brake cylinder pressure according to the difference between the brake command and the total dynamic/friction brake level. Therefore, the pressure regulated is the intermediate pressure, which slows the system response time.
Because of the relatively small control volume of the J-1 relay valve in this arrangement, and the jerk limit or rate control imposed on the brake control signal emitted by the electronic control unit, the application magnet valve exhibits a high cycling characteristic, whereby the J-1 relay valve is operated to develop braking pressure that follows a jerk or rate-controlled buildup-curve having a staircase pattern. While this is desirable in achieving jerk control for optimum passenger comfort, the cycling effect results in a shortened service life of the application magnet valve.
In U.S. Pat. No. 3,398,993, assigned to the assignee of the present invention, there is disclosed an arrangement (as shown here in FIG. 2) in which the brake cylinder pressure is regulated directly via high-capacity type, electropneumatic application and release magnet valves, and thus does not require the interposition of a relay valve to provide the high-capacity requirement of the brake cylinder. Such an arrangement employs a fast-rate choke at the outlet of the application valve to control the maximum rate of brake cylinder pressure buildup, and a pressure transducer between this choke and brake cylinder to provide the electronic control unit with a feedback signal that allows the electronic control unit to regulate the brake cylinder pressure without undue cycling of the magnet valves. Such an arrangement thus reduces the number of pneumatic valve components and lends itself to a normal service life of the application magnet valve. It will be appreciated, however, that since the fast-rate choke is in the line between the brake cylinder and application and release magnet valves, the brake cylinder pressure supply and exhaust is restricted, thus rendering such an arrangement undesirable in terms of employing the high-capacity application and release magnet valves for the emergency and wheel-slip functions. Thus, a relatively large number of pneumatic valve components are required in this arrangement, as well as in the first-discussed arrangement, to provide the service, emergency and wheel-slip control functions.